1. Field of the Invention
The present invention relates to an optical transmitting and receiving module for coupling a light emitting device and a light receiving device with an optical fiber through which a plurality of transmitting light beams and receiving light beams having different wavelengths are transferred.
2. Description of the Related Art
FIG. 5 is a cross section of a conventional optical transmitting and receiving module. As shown in the figure, the casing of this optical transmitting and receiving module can be roughly divided into four blocks, a center block 1, a laser diode (LD) block 2, a photodiode (PD) block 3, and a fiber block 4. Inside the center block 1, an optical demultiplexing filter 5 is mounted. A laser diode (LD) 6 serving as a light emitting device and a lens holder 8 for holding a lens 7 are mounted in the LD block 2. A photodiode (PD) 9 serving as a light receiving device and a lens holder 11 for holding a lens 10 are mounted in the PD block 3. An optical fiber 12 and a lens holder 14 for holding a lens 13 are mounted in the fiber block 4.
A transmitting light beam having a wavelength .lambda.1 emitted from the LD 6 is made parallel by the lens 7 in the LD block 2, passes through the optical demultiplexing filter 5 in the center block 1, is collected by the lens 13 in the fiber block 4, enters an end face of the optical fiber 12, and is transmitted through the optical fiber 12. On the other hand, a receiving light beam having a wavelength .lambda.2 transmitted through the optical fiber 12 is diffused at the end face of the optical fiber 12, is made parallel by the lens 13 in the fiber block 4, is reflected from the optical demultiplexing filter 5, is collected by the lens 10 in the PD block 3, and is received by the PD 9.
In the above-described conventional optical transmitting and receiving module, ideally, the optical demultiplexing filter 5 has an optical demultiplexing characteristic in which only a transmitting light beam having the wavelength .lambda.1 emitted from the LD 6 is passed. Since a part of the transmitting light beam having the wavelength .lambda.1 is actually reflected, the reflected light beam is irregularly reflected at inside walls of the center block 1, passes through the optical demultiplexing filter 5, and undesirably enters the light receiving surface of the PD 9. When the transmitting light beam is detected by the PD 9 in this way, the S/N ratio of a receiving signal deteriorates.